1. Technical Field
This invention relates to radomes and lenses and. more particularly to a radome or lens with two impedance matching layers.
2. Discussion
Electromagnetic antennas, including radar antennas are used under a variety of environmental conditions. Without protection, these antennas become vulnerable to the adverse effects of rain, heat, erosion, pressure and other sources of damage, depending upon where the antenna is used. Radar antennas, for instance, have been used in space-based, airborne, ship-borne and land-based applications. In each of these applications an antenna is subjected to a different set of environmental forces, some of which have the potential to render an unprotected antenna inoperable or severely damaged.
In order to protect an antenna from the adverse effects of its environment, antennas have been enclosed by shells which shield the antenna from its environment. The shielding of the antenna is typically accomplished by housing it within a relatively thin shell which is large enough so as not to interfere with any scanning motion of the antenna. The shielding shells used for radar antennas are typically called radomes.
A particular radome design is required to protect its antenna from the surrounding environment, while simultaneously not interfering with signals passed to and from the antenna and while not interfering with the overall performance of the system upon which the antenna is mounted. For instance, in airborne applications, a radome protects an antenna from aerodynamic forces and meteoric damage, while at the same time allowing radar transmission and reception, and while preventing the antenna from upsetting the aerodynamic characteristics of the airborne vehicle upon which it is mounted. Radomes are employed in ship-borne applications to protect antennas from wind and water damage, and from blast pressures from nearby guns.
Lenses have been used in connection with horn antennas to facilitate transmission and reception of electromagnetic signals. The lens is typically positioned in the path of the electromagnetic signal, and in front of the horn antenna The lens is used to bend or focus the signal, as the signal is transmitted or received.
Of particular importance are the electromagnetic characteristics of materials used in building the radome or lens. Currently, the structures used to produce radomes and lenses possess permittivities that are not equal to that of free space or of the atmosphere. The resulting impedance mismatch can cause reflections at the boundaries of the radome or lens, and can cause distortion and loss in the electromagnetic signal. The adverse consequences of an impedance mismatch become particularly acute when electromagnetic signals are transmitted or received from high angles of incidence with respect to the radome or lens. Attempts have been made in the past to minimize the effects of the impedance mismatch between the atmosphere or the free space that is in contact with the radome or the lens. For instance, prior attempts to match a radome or lens with a permittivity of: EQU .sup..epsilon. randome or lens.sup.=4*.epsilon. 0
(.epsilon..sub.0 being the permittivity of free space) have included a single impedance matching layer between the radome or lens and the atmosphere. This impedance matching layer has typically had a permittivity whose value falls between that of the atmosphere or free space, and the radome or lens. These previous impedance matching designs have shown good performance only when incoming electromagnetic signals have had small angles of incidence. These prior designs have also shown significant sensitivity to signal polarization.